The following invention relates to a construction for a thin-film electroluminescent device, and particularly, to a transparent coating for a thin-film electroluminescent stack that protects the stack from the effects of moisture and electrical breakdown and helps to prevent burnout in the metallization layer.
A major cause of failure in thin-film electroluminescent devices is phosphor burnout. Burnout may result from contamination of the thin-film electro-luminescent stack which contamination may cause front and rear electrode elements to short circuit creating a burnout that can render a portion of the display inoperative. Burnouts may also be caused by excessive current flowing through the top electrode layer which is usually fabricated from transparent indium tin oxide (ITO). Sometimes excessively high current flow in the ITO layer can be caused by poor aluminum step coverage over the ITO/phosphor stack combination.
The etching process for creating a patterned phosphor layer frequently causes the edges of the phosphor stack to be jagged and uneven, particularly in AMEL displays which are fabricated on a silicon substrate. In such displays a metalization layer which is a thin film of aluminum is deposited on the silicon substrate so as to bridge the step from the substrate to the top ITO layer. Thus the top ITO layer becomes a single wide area scanning electrode. Jagged areas of the phosphor layer can cause uneven deposition of the aluminum layer which can in turn lead to high resistance areas or even breaks in the aluminum layer. This in turn can cause a high current flow in such areas leading to burnout. Burnout can not only melt the aluminum but can lead to destruction of the panel's underlying circuitry.
The same problem exists when the TFEL device is in the form of a passive matrix device which employs a parallel set of elongate scanning electrodes. These electrodes are coupled to driver electronics devices by metal wires or straps which must bridge the step from the substrate up to the electrode surface. A jagged phosphor edge could puncture or sever one or more of the bond wires or straps.